Rock bits typically include a bit body adapted to connect to a drill string at one end with one or more legs integrally connected to form the bit body extending from the other end. Each leg typically includes a rolling cutter rotatably mounted on a journal pin extending from each leg. Bearings are typically provided between each rolling cutter and journal pin to promote rotation of the cutter on the journal pin when the bit is rotated on earth formation. Cutting elements provided on the outer surface of each cutter engage and break up earth formation as the bit is rotated.
Rock bits typically further include a lubricant reservoir system for providing lubricant to the bearings to reduce friction and the operating temperature of the bearings and, thereby, increase bearing performance and bearing life. A lubricant reservoir system typically includes a reservoir in the bit body filled with a lubricant and passages provided therein to permit communication of lubricant from the reservoir to the bearings. One or more annular seals are typically provided at or near the back-face of each rolling cutter between the rolling cutter and the journal pin to prevent lubricant from leaking from the bearing area to an exterior of the rock bit. The seals also function to prevent drilling fluid and debris from entering into the bearing area and damaging the bearings.
The durability and effective drilling life of a rolling cutter rock bit depends on numerous factors. One important factor is the effectiveness of the seals used to protect the bearings. Rock bit seals must function for substantial periods of time in harsh downhole conditions involving high pressure, high temperature, and large amounts of abrasive rock particles entrained in the drilling fluid flowing past the seals. In particular, the temperature around the bearing area can become very high due to excessive heat from friction between bearing surfaces, fracturing of rock by cutting, and geothermal conditions underground.
To enhance seal function and increase seal & bearing life, a balance between the internal and external pressures on a seal should be maintained. For example, when a bit is inserted and moved downhole, the pressure on the outside of the bit will increase due to an increase in the fluid column above the bit and higher pressure conditions downhole. Without pressure compensation, pressure on the drilling fluid side of the seal can become substantially higher than the pressure on the lubricant side of the seal, and particulates from the drilling fluid may be pushed into or past the dynamic face of the seal and lead to a rapid destruction of the seal and bearing system. Additionally, during drilling as the temperature around the bearings increases, lubricant in the bit will thermally expand. Without appropriate pressure compensation, including pressure relief, the pressure on the lubricant side of the seal may become excessive and result in an excessive loss of lubricant pass the seal and premature failure of the seal and bearing system.
To avoid such problems and increase seal and bearing life, lubricant reservoir systems typically include a pressure compensation assembly comprising a pressure compensator in the form of a resilient diaphragm positioned in the lubricant reservoir with one side in fluid communication with lubricant in the bit and the other side in fluid communication with drilling fluid outside of the bit. The compensator functions to equalize the pressure of the lubricant in the bit with the drilling fluid outside of the bit so that the differential pressure across the seal during drilling will be minimized. The pressure compensation assembly is typically configured to include a pressure relief structure for the lubricant reservoir system to protect the compensator from exposure to extreme differential pressures that can result due to excessive thermal expansion or overfill in the system. Pressure relief structure typically includes some form of a valve face biased against a valve seat by a bias force provided by a bias member. The pressure relief structure is arranged such that when excessive lubricant pressure occurs in the reservoir system the bias force will be overcome and the valve face will be displace from the valve seat to permit lubricant to vent there between until the pressure differential is reduced to an acceptable level.
In conventional reservoir systems, once a bias member is selected and assembled in the system, the relief pressure of the system is set and cannot be changed. Machining errors and tolerances can cause variation in the relief pressure of a system. As a result, the set pressure at which a particular system will relieve is not know, but rather is considered to fall within a range, such as from 50 to 200 pounds per square inch (psi) depending on the size or dimensions of the bit. Thus, lubricant reservoir systems in different bits or different legs of a bit may be exposed to different maximum pressures during drilling, which can lead to an earlier failure in one of the systems. Additionally, if a different relief pressure is desired for a system, the system will have to be disassembled and different parts introduced or redesigned. This can increase bit manufacturing cost significantly. Accordingly, a pressure compensation assembly having an adjustable relief pressure is desired so that the relief pressure of a system can be changed or adjusted without requiring redesign or the use of different parts in the system.